This invention relates to an exhaust gas-driven turbocharger for an internal combustion engine.
Turbochargers for internal combustion engines generally comprise an engine exhaust-driven turbine which in turn drives an engine air inlet compressor. The turbine and compressor sections may be fixed to a common shaft so that the turbine may directly drive the compressor, and the shaft bearings may be lubricated with oil from the engine sump.
With such an arrangement, there is a known potential for air leakage from the compressor section volute through the shaft bearings and into the engine sump (known as xe2x80x9cblow-byxe2x80x9d). This may cause the sump to be pressurized, with consequent ill effect on the engine. Also, for large industrial engines it is common to prime the turbocharger shaft bearings with oil prior to startup of the engine. With inadequate oil sealing for the bearings, oil can leak on to the back face of the compressor impeller. When the impeller starts to rotate, the leaked oil will be thrown outwards to mix with the compressor air, so entering the combustion chambers of the engine, again with possible detrimental effect on the engine.
In an attempt to solve this problem, an annular metal seal in the manner of a piston ring has been provided to isolate the bearing from the compressor volute. This ring seal is a sliding or xe2x80x9cfloatingxe2x80x9d fit in a bore of a casing surrounding the shaft and projects into a groove in the hub of the impeller to form a sealing interface therewith.
While this arrangement may be acceptable in relatively small turbochargers, it is less satisfactory in larger turbochargers where the boost pressure is high. Blow-by pressure in larger turbochargers forces the ring seal to move axially in the casing bore towards the shaft bearing. This causes the ring seal to contact a side face of its locating groove. While this is desirable for establishment of a good seal, it causes the ring seal to wear until it eventually abuts a shoulder in the casing bore (to provide a limit to further axial movement of the ring seal). At this point, further wear of the side face of the ring seal against the side face of the groove will tend to lead to the formation of a gap between the ring seal and the side face of the groove, reducing the effectiveness of the seal.
According to the invention, there is provided a turbocharger for an internal combustion engine, comprising:
a casing defining a bore,
a bearing housed within the bore,
impeller shaft means mounted in the bearing for rotation within the bore, and
a floating ring seal located in the bore adjacent the bearing and extending radially into a circumferential recess in the shaft means, the ring seal being subject in use to a pressure differential there across which urges it to slide along the bore in a predetermined axial direction such that it bears against a side face of the recess to form a sealing interface therewith, the ring seal being subject to wear against the side face due to relative rotation therebetween, and a shoulder being provided in the bore to limit excessive axial sliding of the ring seal due to wearing away of the ring seal against the side face of the recess. The sealing performance is improved and the rate of wear of the ring seal is reduced by providing the ring seal with at least one inwardly-facing annular groove and providing the recess with a circumferential rib corresponding to each groove, each rib being receivable in a groove, whereby under the pressure differential a side of each groove forms a sealing interface with a side of the corresponding rib.
As the side face of the ring seal wears, it moves towards the bearing but the wear is shared across side faces of the grooves as well, reducing the rate at which wear occurs. Compared with the prior art, the provision of one or more extra seal interfaces in series with the interface between the ring seal and the side of the recess, means that each interface is required to accommodate only part of the overall pressure differential, and therefore each does not have to be as efficient as a single interface for a given total pressure drop. Over time, seal wear will continue until the ring seal contacts the shoulder in the bore, whereupon small gaps will form equally at the grooves/ribs and ring seal/recess interfaces. However, the combination of these sealing interfaces will still be more efficient than the ring seal/recess interface on its own, because of the sinuous nature of the leakage path and the presence of oil in the gaps.
The shaft means in which the recess is provided may comprise a central hub of the impeller.
The ring seal preferably has a plurality of inwardly-facing annular grooves, the recess being provided with a corresponding plurality of circumferential ribs. For example, two annular grooves may be provided, with two ribs to be received into them. The ring seal may be formed of metal, for example cast iron, the shaft being steel, preferably nitro-carburized. In an alternative embodiment, the ring seal is formed of a plastics material.
To facilitate fitting of the ring seal on to the shaft, it is suitably split in the manner of an engine piston ring. For example, the split may have a dog-leg configuration, extending over half the width of the ring, then circumferentially of the ring for a short way before extending across the remainder of the width of the ring.
The air/oil seal used in the turbocharger of the invention extends the working life of the turbocharger by maintaining satisfactory sealing performance in both directions for a much longer time than conventional ring seals. At the same time, the simple construction of the ring seal ensures that the cost of manufacture of the turbocharger is not increased significantly.
The invention also provides an internal combustion engine fitted with a turbocharger in accordance with the invention.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.